This invention is directed to a radio frequency (RF) choke, and more specifically concerns a choke for separating an AC power wave from a broadband signal, where both are carried on the same conductors.
It is common in CATV distribution systems to use a broadband signal (e.g. 50 to 1000 MHz) to carry the various channels and other information to the subscribers and another broadband signal (e.g. 5 to 30 MHz) to carry information from the subscribers to the cable distribution station, and 60 Hz single phase power to operate amplifiers and other devices located at various points on the cable system. These three waves are all carried on the same conductors, e.g., the center conductor and braid of a coaxial cable.
It is a common practice to use an RF choke to separate the single-phase AC power from the broadband RF signals at points along the cable where the RF signal is to be processed in an RF device. After the device, the AC power is recombined with the broadband signal, and an RF choke is also employed at this position.
The AC power has a current magnitude of 12 to 15 amperes at 60 volts. On the other hand, the broadband RF signal has a low peak voltage, e.g. 0.3 volts. These chokes serve to isolate the RF device from the AC power. However, in doing so the chokes should not permit any significant amount of the RF broadband signal to pass through the choke on the AC power path, as a significant signal loss will occur.
A commercial RF choke is typically constituted by a number of turns of magnet wire wound upon a ferrite coil form. A resistor can be connected in parallel wit a portion of this coil, e.g., from a preselected turn to one of the lead wires, to serve as a shunt. This parallel resistance reduces the impedance of the RF choke. There is an effective capacitance between turns of the wire coil, which produces a self-capacitance that combines with the coil inductance to produce an LC resonance. Typically, such resonances unfortunately often lie within the band of the broadband RF signal. The effect of the shunt resistance is to reduce the Q of the LC resonance, thereby blunting the sharpness of any in-band resonances.
Reduction in the number of turns of the choke can push any LC resonances above the passband, but this reduction will also result in a reduction in inductance, limiting the suitability of the choke at the 5 MHz low end of the band.
The presence of the shunt resistor in the above-described choke also reduces the signal impedance to ground, thereby increasing the signal loss.
One attempt to remedy the above problems is described in U.S. Pat. No. 4,394,631 to Pavlic. In that approach, an RF "resistor" is formed of a bead of ferrite disposed over the wire at a predetermined turn on the coil, and a second resistor is connected in parallel to a portion of the coil between a preselected turn and a lead wire. The effect of the series "resistor" is to push the choke LC resonance up above the pass band. However, for a number of reasons, the pass band is limited to a maximum of 400 to 500 MHz.
The heavy current (12 to 15 A) through the choke has a tendency to saturate the magnetic material of the bead, so that its effectiveness on the RF signal is reduced. Also, no effort is made to reduce the cumulative capacitance between turns. In fact, a glue or cement used to hold the coil onto the form actually serves as a dielectric, thus increasing the self-capacitance and further emphasizing any LC resonances. Consequently, it has been impossible to eliminate resonance in the range between about 500 MHz and 1000 MHz.
However, this is a significant band of frequencies, as TV cable systems must be able to carry RF signals in the entire range between 5 MHz and 1000 MHz.